In the preparation of an oil drop dispersion containing a hydrophobic photographically useful substance, such as couplers, ultraviolet absorbents, and high boiling organic solvents, a commonly and widely employed method comprises dissolving such a hydrophobic substance in an auxiliary solvent and emulsifying and dispersing the solution in an aqueous medium as disclosed in U.S. Pat. No. 2,327,027. However, the dispersion prepared by this method is apt to undergo precipitation of the hydrophobic substance or change in particle size of the dispersed lipophilic fine particles when it is preserved in the dissolved state or at a low temperature.
Other available methods include a method of impregnating a hydrophobic substance into a latex polymer. For example, there is a method in which a hydrophobic substance, e.g., an oil-soluble coupler, is dissolved in a water-miscible organic solvent and loaded on latex particles obtained by emulsion polymerization as disclosed in U.S. Pat. No. 4,203,716; and a method in which a hydrophobic substance is dissolved in a substantially water-immiscible organic solvent and loaded on latex particles by emulsifying and dispersing as disclosed in JP-B-51-39853 (the term "JP-B" as used herein refers to an "examined Japanese patent publication").
These methods have difficulty in uniformly loading the hydrophobic substance on latex particles with good reproducibility.
WO 88/00723 and JP-A-63-264748 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") disclose a method using a dispersion prepared by dispersing a mixed solution having dissolved therein a hydrophobic substance together with a water-insoluble and organic solvent-soluble polymer. According to this method, the lipophilic fine particles have improved stability, and the hydrophobic substance per se in the dispersion used in a photographic light-sensitive material exhibits improved stability to light and heat. It turned out, however, that the method disclosed in WO 88/00723 and JP-A-63-264748 has the following disadvantages in practical use. Firstly, simultaneous dissolving of a hydrophobic substance and a polymer requires a longer time than in the case of separately dissolving each of them. Secondly, since a mixed solution comprising a hydrophobic substance and a polymer exhibits high viscosity probably attributed to a strong interaction between them, it is hardly dispersed and liable to produce coarse particles. As a result, color developability or coating properties tend to be reduced. Moreover, when the resulting dispersion is preserved in a dissolved state or at a low temperature, the lipophilic fine particles are sometimes observed to become coarse. Thirdly, stability of the hydrophobic substance in lipophilic fine particles to light and heat is fairly improved, but there is a need for further improvement for practical use.